The invention pertains to a method for level measuring by means of an oscillating fork that is excited to oscillate by an oscillator circuit and to which an identifying element is assigned; namely, according to the characteristics of the preamble of claim 1.
The invention also pertains to an arrangement for level measuring by means of an oscillating fork that is excited to oscillate by an oscillator circuit and to which an identifying element is assigned; namely, according to the characteristics of the preamble of claim 9.
The resonant frequency of oscillating forks that, for example, are used for level measuring is subject to scatter due to casting and processing tolerances. The electronic circuit that evaluates this resonant frequency needs to be adapted to this frequency scatter by means of suitable measures.
In this context, a first state of the art is described in DE 42 32 659. In this case, an identifying element is assigned to the oscillating fork, with the identifying element forwarding the value of the oscillating fork frequency in a coded fashion to the frequency evaluation stage of the sensor electronic. The frequency evaluation stage adapts its switching point to the coded value such that all sensors have the same filling level response height despite different fundamental frequencies of the oscillating forks.
A second state of the art is described in DE 198 24 370 A1. In this case, two identifying elements are assigned to the oscillating fork, with the two identifying elements respectively adjusting one reference band filter in the sensor electronic, and with said reference band filter being connected to the regenerative oscillator circuit instead of the oscillating fork. The generated reference frequencies are transmitted to the evaluation device that adapts its own frequency switching point to the transmitted reference value such that the same filling level response height is always achieved despite different fundamental frequencies of the oscillating forks.
The disadvantage of both aforementioned methods can be seen in the fact that although the frequency evaluation stage is adapted to the manufacturing tolerances of the oscillating fork, the regenerative oscillator that serves to excite the oscillating fork undergoes no adaptation.
In practical applications, the fundamental band filter contained in the regenerative oscillator was until now designed to be so broad with respect to its frequency response that it covers the fundamental frequencies of all oscillating forks that lie in the respective range of manufacturing tolerances.
However, this leads to problems in miniaturized oscillating forks that, for example, only have a prong length of 40 mm. Due to their inferior oscillatory properties, oscillating forks with such a short length increase the demands on the regenerative oscillator. If the fundamental band filter is designed to be excessively broad, problems in the oscillation build-up of the fork occur when the power supply is switched on. If the fundamental band filter is designed to be excessively narrow, the oscillating frequency of the fork is excessively distorted by the regenerative oscillator if the fundamental frequency values of the oscillating fork respectively lie at the upper or the lower end of the scatter range. This inevitably results in the oscillating fork being excited next to its resonant frequency. Depending on the tolerance value of the fork, this may, under certain circumstances, result in prevention of the output of the full level message or the empty level message, respectively.
The aforementioned problems occur, in particular, with oscillating forks that are coated with plastic or enamel for reasons of corrosion protection, as well as with forks that are polished to a mirror finish and used for food applications. In addition to the broader scatter ranges of their fundamental frequency, forks of this type frequently have an asymmetric mass distribution between the prongs of the fork which is caused by the coating and polishing processes, respectively. Analogously, oscillating forks for high-temperature applications increase the demands on the regenerative oscillator in order to be able to cover the entire temperature range with constant measuring characteristics.
The method according to the invention is based on the objective of developing an arrangement that makes it possible to compensate mechanical tolerances with electronic means.
With respect to the method, this objective is attained with the characteristics of claim 1.
With respect to the device, this objective is attained with the characteristics of claim 9.